Polarization-dependent loss (PDL) is an impairment in a fiber optic channel that leads to a loss of performance, and, in contrast to chromatic dispersion (CD) or polarisation mode dispersion (PMD), cannot be fully compensated.
PDL usually originates in components like optical isolators, circulators, optical amplifiers or WDM (wavelength division multiplex) couplers. The PDL of a single element is usually low. However, depending on the time-dependent polarization rotation in between the PDL elements, the instant PDL value can reach rather high values. For N PDL-elements in the system, the mean PDL over time is given by A. Mecozzi, A. Shtaif, “The Statistics of Polarization-Dependent Loss in Optical Communication Systems”, IEEE Photonics Technology Letters, vol. 14, pp. 313-315, 2002:
            (      PDL      )        =                                        8            ⁢            N                                3            ⁢            π                              ⁢      Δα        ,where Δα is the PDL[dB] per element and (PDL) is the expectation PDL value. It was shown that the probability distribution of PDL is Maxwellian for a large number of PDL elements. Here, the probability for the PDL to surpass a certain threshold is P(PDL>2(PDL))=1.7%, and P(PDL>3(PDL))=4.2×10−5, corresponding to 22 minutes per year. Assuming an optical link with 20 spans of fiber and 20 spans of optical dispersion compensating elements, 5 PDL elements per span with Δα=0.15 dB, the mean PDL would be 1.95 dB. This means that the instant PDL can on average exceed almost 6 dB in 22 minutes per year.
Depending on the polarization of the input signal, a worst and best case performance can be defined, as given by T. Duthel et al. for polmux signals, “Impact of Polarization-Dependent Loss on Coherent POLMUX-NRZ-DQPSK”, San Diego, Calif., OThU5, 2008. If the channel is known at the transmitter, the signal can be aligned to yield best case performance. This would however require a feedback channel, which is usually not given in an optical system. Furthermore, it is possible to scramble the state of polarization (SOP) of the signal, leading to a time-averaging over different performance levels depending on the polarization alignment of the signal in regard to the PDL axis. The performance of SOP-scrambling leads to a limited performance improvement of 0.5 dB at 6 dB PDL for polarization multiplexed signals. So far, no elaborate PDL mitigation technique for coherent polarization multiplexed systems was presented.